GREEN BAY - The revolving door at center continues for the Green Bay Packers due to injury.

This time around, starter JC Tretter will be handing the job back to Corey Linsley because a knee injury he suffered against the Atlanta Falcons is going to sideline him several weeks, a source said.

The Packers were lucky the injury wasn't worse given the shot Tretter took to his right knee on the second-to-last play Sunday at the Georgia Dome. Tretter was able to play the final snap, but he needed help getting to the locker room after the game.

Judging by the way Tretter got hit from the side, he might have suffered an MCL sprain. If that's the case, it's not something that requires surgery and is usually a 2-to-6-week injury. Many players are able to wear a brace that allows them to return sooner.

The source didn't say whether Tretter suffered an MCL sprain but said it wouldn't require surgery.

The injury means the Packers are almost certain to activate Linsley from the physically unable to perform list. Linsley, who missed all of training camp and the first six weeks of the regular season, began practicing after the New York Giants game.

Once he started practicing, a three-week window began in which the Packers could see where Linsley was physically. At the end of three weeks, they would have to either activate him to the 53-man roster, put him on injured reserve or cut him.

Fortunately for the Packers, Linsley will have had three full weeks of practice heading into the Colts game. There shouldn't be much drop-off at the position except for whatever rust Linsley has to shake off. Linsley has been doing extra work to get back into playing shape given how much time he has missed.

"We've been doing extra work with Corey all along," coach Mike McCarthy said. "I think it first starts in the strength and conditioning aspect of it. He's a pro in his approach, so his conditioning I would say is at a very high level right now.

"But there's nothing like football shape. We understand that. So the padded work opportunity in the post-practice things, he's been putting in the extra time."

Linsley became the Packers' starter in 2014 after Tretter injured his knee in an exhibition game. For the next two seasons, he held the starting position and was extremely consistent.

He missed three games with an ankle injury last season and Tretter filled in for him. Now the situation is reversed and Linsley will be taking over for Tretter.

Linsley is a very effective pass blocker and probably more consistent than Tretter in that area, but Tretter is a better athlete and very good in the run game.

The Packers don't have to make a move in order to make room for Linsley because they cut running back Knile Davis on Monday.